Asthma is a disease of variable airflow obstruction, bronchial hyperresponsiveness, and airway inflammation. Although most patients respond to conventional therapies, a substantial percentage of patients have severe or refractory asthma manifested by frequent exacerbations and irreversible airflow limitation and airway inflammation despite maximal medical therapy. Patients with severe asthma also disproportionately utilize healthcare resources, and have more adverse effects from high doses of medication. The search for new therapies for severe asthmatics continues.